1. Field of the Invention
The invention relates generally to survey equipment and navigation systems and more specifically to the use of pseudo-random number code modulated light beams and receivers to detect and range the reflected light with navigation receiver integrated circuits.
2. Description of the Prior Art
Land surveyors have used optical equipment to measure distances between points for a century or more. Electronic distance measuring devices have been in use for many years that use the contrast in phase changes of several beams of light or the time it takes a pulse of laser light to reach a target and return to measure difference. Such techniques use the frequency space or time space to make such measurements.
The accuracy of electronic distance measurement equipment is derived from an internal reference frequency source, e.g., a crystal oscillator. But such crystal oscillators can drift over time and with age. Exposure to extreme environments can also upset delicate calibrations of the reference frequency source, both short term and long term. Therefore, EDM equipment should be regularly calibrated by using it to measure a known length.
Long-range electronic distance meters, e.g., ranges over five kilometers, typically use microwave signals for measurement. Short range electronic distance meters often use infrared light. See, Rueger, J. M., Electronic Distance Measurement--An Introduction, Springer Verlag, Berlin, third edition, 1990. Both the long-range and short-range EDM's use pulse or phase comparison methods to determine the distance between instrument and a remote target. However, the phase comparison method is more commonly used for survey instruments.
The pulse technique is based on timing the signal travel time to and from a distant reflector. The velocity of the signal is assumed to be known. For phase comparison, the phase difference of signals is observed at several frequencies. The unambiguous distance between the target and the instrument is resolved using phase difference observations. But in all cases, the basis for measurement precision depends on the accuracy of the stand-alone reference frequency source.
Ingensand, et al., describe in U.S. Pat. No. 5,233,357, issued Aug. 3, 1993, a terrestrial surveying system that has an electro-optic total station connected to a satellite position-measuring system. A GPS receiver can be mounted directly to the total station and thereby determine the position of the total station. The angle and distance from the total station to aiming points "can be determined by conventional surveying methods". Coordinate transformation from the satellite system to a terrestrial measurement is done by a computer.
Such total station is described in more detail in U.S. Pat. No. 5,077,557, issued Dec. 31, 1991, to Ingensand. An ultrasonic electro-optical range finder is included with a GPS receiver in each total station. Such total stations allow points to be measured with the combination of the range finder and GPS receiver that are not directly accessible to the GPS receiver alone. The coordinates of such points to be surveyed are determined unambiguously by means of an arc intersection method. Two or more total stations can be used on the same survey point to advantage, e.g., to increase confidence by redundant measurements.